Radiofrequency amplification system



OCt. l1, 1932. E, W FEAR|NG RDIOFREQUENCY AMPLIFICATION SYSTEM FiledJune l, 1927 A TTORN Patented Oct. 11, 1932 UNITED STATES ATENT FFI-QE'EDWARD w. FEARING, or EAST ORANGE, naw JERSEY RADIOFREQUENCYAMPLIFCATION SYSTEM Application led Tune 1,

This invention relates to the neutralizing of capacity couplings betweenthe grid and plate circuits of a vacuum tube, which couplings result inan undesirable reaction of the plate circuit on the grid circuit. y

More particularly this invention has for its v object to provide a moresimple, efficient and exact neutralizing circuit arrangement which maybe readily adapted for use in any type of vacuum tube and which preventsthe feed bach of energy from output terminals'of the tube to the inputterminals of the tube. This circuit may be readily adapted for use withany form or type of vacuum tube which has been heretofore used without aneutralizing arrangement for neutralizing the feed bach effects. Thismay be done with a minimum of changes and a maximum of efficiency in theresultJ obtained.

lt is well lrnown that a. vacuum tube operates on the principle ofcapacity effect within an enclosed envelope in which a substantialquantity or all the gas has been removed. IThe purpose of the removal ofthis gas has been to enhance the capacity and electronic effectslproduced by electrical discharges. Usually these discharges areincreased by the use of a filament or other heated body to throw offelectrons in the direction of the elements.

Where a plurality of elements are used within an enclosed and evacuatedenvelope a plurality of effects occur, and oftentimes some of theseeffects are disadvantageous to the result desired because they increasethe amplification of the circuit to such a point i that the tube becomesoverloaded.

Ordinarily within the envelope of a three or four element vacuum tube acontrol element has been used in capacity relation to a second elementwhich is used in the load circuit. The variations of conductivitybetween these two elements necessarily vary the capacity between one ofthese elements and third or fourth element thereby affecting; therelative ratio of voltage tension between them and thereby causingvariation in amplification. y n

lt is an object of this invention to retain sufficient feed-backeiectsof a vacuum tube 1927. Serial N0. 195,829.

to produce a reasonable amplification and tow counteract the feed backeffects of the vacuum tube such as would produce an over-abundance ofamplification caused from feed-back from the output circuit to the inputcircuit of the vacuum tube and also to substantiallyfmaintain thisamplication constant regardless of frequency.

Another object of this invention is to provide a vacuum tube with aneutralizing cir- 60 cuit coupled to both the input and the outputicircuits of the vacuum tube, and having a ratio of coupling therewithwhich dii'l'ers from unity whereby the amplification is kept more nearlyconstant in respect to frequency. rlliis difference is determined by theratio off current of one of the circuit coils in proportion to tnecurrent in the neutralizing coil. rl`his` invention relates to theapplication of a neutralizing circuit which is coupled with, but has a.different number of turns in its; coupling coil than there is in theinput coil of the vacuum tube. This same proportion, having a ratiodiffering from unity or one hundred percent coupling, would also bepresent in connection with the output coil and the neutralizing coilcoupled thereto.

This invention relates also to the application of a neutralizing circuitto a vacuum tube in which the voltages and current induced therein aredifferent from the feed back voltages and feed-back currents whichcirculate in both the output and input circuits simultaneously,differing accordingto the ratios of couplings between the input and theoutput coils to those of the input and-f3 output neutralizing coilscoupled therewith. This ratio may be adjusted to the point where anyvariation of coupling due to variation of frequency may be compensatedfor.

Another object of this invention is to em?? body the desirable featuresof a. separate neutralizing` or counter feed-back circuit with aplurality of vacuum tubes. Thus a maximum elhciency results and properadjustment for` the preventing of undesirable feed-backs at" onefrequency will automatically produce a neutralization of inherentfeed-backs at all frequencies at which the apparatus is used.

A further object of this invention is to improve the operation of avacuum tube when used with a second circuit coupled therewith and toovercome the difficulties encountered due to variations in the amount ofcoupling caused from variations of frequency. It has been found'that inan ordinary coupling arrangement different ratios of energy couplingsresult when different frequencies are impressed upon the couplings. Itis an object of this invention therefore to reduce such variations incouplings to a minimum by proper arrangement and proportioning ofcircuits, and to compensate for their variation.

Further and more definite objects will be observed in connection withthe following specifications, claims and drawing in which,

Figure 1 represents a simple schematic arrangement of an ordinary threeelectrode amplifier and the simplied neutralizing circuit coupledthereto;

VFigure 2 shows an analytical circuit arrangement corresponding to thediagram in Figure 1;

Figure 3 shows the application of my neutralizing circuit to apush-pull7 amplifier;

Figure l shows a modified neutralizing circuit which should be appliedto Fig. 3 in place of the one shown therein;

Figure 5 shows a still further adaptation of my arrangement with apush-pull amplifier;

Figure 6 shows a method of winding the wires which form the couplingcoil;

Figure 7 shows another method of obtaining this same result where aplurality of coils yare desired to be wound concurrently and having amaximum of coupling efficiency.

As is well understood by those skilled in the art, ordinary neutralizingcircuits are ineffective in certain conditions and at such frequencieswill not neutralize to sufficient extent the energy which finds its wayfrom the output circuit back into the input circuit unless additionaladjustments are made besides those which occur during the act of tuning.Difiiculty is encountered when mechanically arranging the values andparts so that a complete neutralization will occur at all frequencies atwhich the apparatus is desired to be used.

Such a diiiiculty occurs in a circuit of the vacuum tube 1 with inputcircuit 2 composed of a coil or impedance 8 and output circuit 4,composed of impedance 5. The filament 6 is of the customary type forminga common return in this ease for the output circuit 4 and the inputcircuit 3. A source of potential is shown at 7 in the output circuit.Capacity 8 represents the grid-plate capacity.

Feed-back is prevented by coupling the circuit 9 to the vacuum tubecircuit. Coils 10 and 11 are coupled with coils 3 and 5 respectively anda series condenser 12 is prop-n erly proportioned to capacity 8.

n order to overcome the dilhculty encountered when using one of thecoils 3 or 1() to constitute a tuned circuit and thereby establish causefor variations for energy circulating through these circuits, the tunedcircuit 14 is coupled, as nearly as possible, equally to these twocircuits. The tuned circuit 14 is made up of a variable capacity 15 andthe coil 16. It might well be made up of a variable coil 16 and a fixedcapacity 15, or both elements might be made adjustable. In any event itis desirable to have coils 3, 10 and 16 as closely coupled as possible.A method of obtaining this result is to be described further on. Thesame is true of the coupling circuit 17 which is shown as coil 18 andvariable capacity 19 coupled as closely as possible and equally to coils5 and 11. From these two tuned circuits and the manner of coupling tothe vacuum tube and neutralizing circuit it will be seen that anyenergies existing in tuned circuit 17 will affect equally the coils 5and 11 to thereby feed back energy in an equal amount to coils 2 and 10where each will be substantially ceunteracted by the other. The samewill be true of any energy which is stored in the circuit 14 which mayattempt to react through the capacity 8 on energy or voltage applied tothe coil 5 to cause undesirable oscillations,

The terminals 2O of coil 16 may be connected to the source ofalternating current energy desired to be amplified either in the form ofan antenna or in the form of another amplifying circuit or other device.rlhe terminals 21 of the tuned circuit 17 may be connected to apparatusin which it is desired that the amplified signals will circulate.

In order to accomplish this result where all frequencies within a widerange may be impressed upon the amplifier, it is necessary that thesecircuits, be correctly proportioned in all their elements and details.All of the elements must be proportioned in proper ratio with eachother. Heretofore it has been considered that it is only necessary tohave one of the coils in the amplifier with relation to one of the coilsin the counter feed-back circuit in the correct ratio. It has been foundthat this prior method of proportioning the circuit is inaccurate anddoes not hold for all conditions and all frequencies which are impressedupon these circuits.

These two circuits may be proportioned so that all of the differentelements will have exactly the same value, that is, the coil 2 will beof exactly the same diameter and the saine number of turns as coil 10,coil 5 will have exactly the same diameter and the same number of turnsas coil 11, and the capacity 12 will be exactly the same size as theinherent capacity 8 between the grid and plate of the vacuum tube 1.This provides a means f of neutralizing the inherent effects betweensubstantially unity. In order to neutralize for. all frequencies, directconductive relations between the two circuits may be eli1ninated and thetwo circuits may be separately coupled as shown in Figure 1. Thisprovides means whereby, the effects of the tuned v burn out thefilament.

circuits on the capacity of the plate and grid of the vacuum tube arethe same as the effect of these circuits upon the condenser 12 in thecounter feed-back circuit and therefore undesirable oscillations areprevented in thevacuum tube circuit. This permits the variation of allthe constants of the two circuits in the correct ratios'.

It isalso obvious from this arrangement that no direct contact is madewith either the i input circuit or the output circuit and that energy istaken off from both these circuits by means of inductive couplings.Therefore, in case the condenser 12 breaks down or there is a shortcircuit between any part of the neutralizing circuit, no difiiculty willbe encountered such as the burning out of the filamentvas would be thecase particularly when the filament resistance switch 22 is in the .offposition as shown in Figure 1.

It is necessary that the insulation between coils 3 and 10, as well asthe insulation between coil 5 and coil 11 be broken down at the sametime in the present arrangement to This is very unlikely to happen andtherefore thisradditional fac tor of safety results. Where unusuallyhigh voltages are used in place of the battery 7, an additional capacitycould be added in the connection23 which, when added to the capacity atl2 would give atotal capacitance effect in the proper proportion to thecapacitance 8 of the vacuum tube.

In Figure 2 is shown a simplified circuitof the arrangement in Figure 1.All of the equivalent parts bear the same indicating numbers except thatthey are primed.

The proper relation between the coils and the constants of the variouscircuits is determined as follows: Assuming that all coils are the samediameter and of the same resistance wire, with the coils spaced the samedistance apart, and preferably as close as possible, then the number ofturns in coil 3 might be represented by A, the turns in coil 5 by B, andthe inherent capacity between these two coils by c; this would alsoinclude other external capacities.

vCoil 10 would be represented by a, coil 11 by b, andk capacity 12` byc2. Then the relation between capacity c2 and the capacity c1 would beequal to the following expression and not simply Lemie This is truebecause the energy factor or measure of ability to store energy of alcircuit containing capacity and inductance in parallel for a givenfrequency is approximately the capacity times the inductance. Thereforeto get the two circuits (the vacuumtube circuit and thecounter-feedbaclr circuit) to have the same stored energies,

but other factors enter into the circuits namely the transformationratios of the couplings. Thus, power is transmitted from B to b With thevoltage ratio of and from a to A with the voltage ratio of A and thisvoltage ratio is the direct measure tio is the direct measure of thethrottling ef- A fect of the condensers.

Therefore A+BBa c A+BBa-c a+b X 1 XAOOT +1 XAXb-o In other words thetotal impedance of the vacuum tube circuit to the total impedance of theneutralizing circuit times the ratio of the second coil in the vacuumtube circuit to the iirst coil in the vacuum tube circuit times the irstcoil in the neutralizing circuit to the second coil in the neutralizingcircuit should be equal to the neutralizing capacity divided by thevacuum tube circuit capacity.

Theoreticaily these values would simply relate to the inductance of thevarious elements but practically it mav be assumed that the coils, ifthey are wound in a manand increasing the-capacity to such a pointYthat; the regeneration of the circuit is substantially the same for allfrequencies, thus keeping the amplification and' feed-back constantregardless of variations in inductive couplings and'increased capacitivefeed-back pull or tandem amplifier circuit. Such an amplifier circuit isshown in Figure 3 where the vacuum tubes 24 and 2,5 are connected withtheir Vinput coils in the form of a single coupled coil 26, the halvesof which are as closely coupled as is possible and the terminals ofwhichare arranged so that one of the tubes 24 or 25 is energized duringeach alternate half cycle. The ouput coil 27 is of the same nature asthe input coil and is connected at its middle tapv 28 through a sourceof high potential 29 connected to the return to the filament 30. Thefilaments are energized by the customary batteries 31 which have acontrolling switch and resistance as in the case of Figure 1.

Connection ismade for the input to the amplifier by means of terminals32 to a coil 33 which may be closely coupled to both halves of coil 26and neutralizing coil 34 similar to that described in Figure 1. Theoutput tothe ampliers mav be connected through the terminals 35 of anoutput coil 36 coupled equally and as close as possible to the coils 27and 37 of the neutralizing circuit.

The equivalent capacities of the vacuum tubes are represented by thecondensers shown in dotted lines as 38 and 39. A counter-balancingcondenser 40 is designed so that it bears the proper relation betweenthese two'condensers and the coils of the respective circuits such as isdescribed in connection with Figure 1. Assuming that the upper halfofvcoils 26 and 27 are equal to the lower halves and to the coils 34 and37 respectivelyV thenthe capacity 40 should equal the sum of thecapacities 38 and 39.

In order to apply the formula given in connection with Fig. 1 it must beassumed that the two halves of coils 26 and 27 are substantially equaland in the proper proportions to the constants of the neutralizingcircuit. The only difference in this amplifier has been found to be,instead of neutralizing a single tube by the capacity 40 the amount ofenergy transferred by this neutralizing circuit or the capabilities totransfer energy must be equal to the capabilities to transfer energy ofboth the circuits, 38, 26, 30, 29, 27 and 39, 36, 30, 29 and 27. Ifthese two circuits are substantially identical and equal they willtransfer twice as much energy as a single circuit and it has been foundthat this energy is transmitted in the same direction and that thereforea single neutralizing circuit is sufficient to compensate for both.

y In case high potentials are applied to the output circuit of such anamplifier a neutralizing circuit such as is shown in Figure 4 may beused where 41 and 42 are the coils which are'closely coupled to theinput and output coils respectively and the capacity 43 plus 44, otherthings being equal, are the same as that of the condenser 40 shown inFigure 4. This is also an advantageous arrangement when used with highfrequencies for the salie of symmetry.

' I-Ieretofore it has been considered that vacuum tubes were neutralizedby the fact that voltages equal and opposite in effect were applied at asingle point to prevent the flow of current. Practically such acondition never occurs because of leakages and other imperfect `use ofinstrumentalities. That usually does occur is a neutralization by thebalancing of an actual fiow of current and of applied energy in the formof variation iii-magnetic field within the coils connected to theamplifiers. There is a balancing of energy rather than a balancing ofpotentials or a simple balancing of current. This balancing of energymust be maintained in all corresponding parts of the circuits.

A particular application of this principle to a push-pull amplifier isshown in Figure 5 where the input may be connected to the terminals 45of an input coil 46 coupled equally with two halves of the coil 47across which is connected a tuning condenser 48 if desired. A commonreturn is indicated at 49 for both vacuum tubes through a high potentialbattery 50 and the output coil 51 for both vacuum tubes arranged asdescribed in connection with Figure 4. Here both vacuum tubes 52 and 53are neutralized by the circuit coupled thereto through the condenser 54.The two feed-back circuits through the vacuum tubes are as follows:vacuum tube 52, coil 47, return 49, battery 50 and coil 51; vacuum tube53, coil 47, return 49, battery 50 and coil 51. The counter feed-backcircuit which would balance these two feed-back circuits is as follows:condenser 54, upper-half of coil 51, battery 50, common return 49 andlower-half of coil 47.

The fact that these two `halves of coil 47 and 51 are as closely coupledas possible prevents any variation of effects between the two vacuumtubes 52 and 53, to a substantial degree. In addition to this tightcoupling between these two coils it may be noted that a comparativelylarge circulatory current is flowing from the coil 50 and into thecondenser 48 so that the comparatively small feedback currents havelittle or no effect in producing a time variation of voltages applied tothe end of this coil 47. This aids to overcome any deficiencies in lackof coupling between the two halves of coil 47.

It may also be noted in connection with coil 51 that during eachalternate half-cycle a substantially large load current flows throughthis coil and to the battery 50. This load current causes a largepotential to be applied and a large flux to flow through theconvolutions of this coil. The percentage of change produced on this uxand therefore the percentage of change in this voltage at which the eneris being transmitted between the feedack and the counter-feedbackcircuit as related to this coil 51 is therefore relatively small andthis effect enhances and increases the eect of coupling between the twohalves of this coil.

The output connection may be made through the coil 55 coupled as closelyas possible and equally to the two halves of the coil 51 as described inconnection with some of the previous modications.

It will be noted in connection with these modifications described thatthe coupling .coil or coils are so related `as to provide as close aspossible to one hundred percent. coupling. It has been found that by acarefully designing of these coils that a coupling of as high apercentage as 95% or possibly slightly higher in eXtreme cases has beenproduced. To express it another way a coupling in which the leakage hasbeen reduced to a practical minimum has been accomplished by properproportioning of the parts and improving the mechanical arrangey ment.

These coils may be advantageously constructed of a triple strandconductor such as is show nin Figure 6 composed of strands 56, 57 and58, braided, twisted or otherwise combined into a single cord or cable.If these strands are wound around together in va regular manner and asclose as possible, preferably over a cylindrical form, there will resulta coil having as little leakage between each section as practicable. Thetwisting or braiding of wire aids in the preventing of electrostaticcoupling between turns to a considerable extent, it being understoodthat electrostatic coupling between turns permits a leakage of currentbetween turns and there- I fore a leakage of magnetic flux betweenturns.

Another common and simpler method is shown in Figure 7 where conductors59, 60 and 61 are combined together in a single cord or cable 63 and arewound in the form of a coil as above described.

It can be seen from thisdisclosure that it is practically impossible toobtain a coupling having 100% efliciency or of a coupling factor ofunity.Y Therefore, there is always a transference of energy in any typeof neutralizing circuit which is applied-to a vacuum tube and unlessthis energy finds a path which is of equal resistance, impedance andcapacity, an over or under balance of the vacuum tube circuit results.

This over or under balancing can only be obviated by designing thecircuit on which energy is fed back as nearly as possible similar tothat which counter-feed-back energy is conducted. I-Ieretofore this.identity of circuits has not been maintained it being thought necessaryto produce only equal and opposite voltages.

As explained above these equal and opposite voltages do not entirelyneutralize the vacuum tube and particularly at all frequencies. Thisneutralization can only be accomplished when the parts and elements areproportioned according to the formula specified above. It has also beenfound that it is not entirely necessary to maintain a high value ofcoupling between the various coils in case the proper ratios of theelements in the circuits are maintained, through which the feed-back andcounter-feed-back currents and energies circulate.

It has been found in practice that suiicient coupling results betweentwo or more coils, for the purposes described herein, if each one of thecoils is simply wound on one of a plurality of concentric and tightfitting cylinders, either in the form of cardboard, bakelite or othermaterial having thin rigid walls. Any coupling which approaches in valuethe range of 35 to 85% is usually suiiicient in most low powered radioreceiving circuits to produce the proper counter-acting effect of thefeed-back current, substantially throughout the broad-cast range.

It is not intended to be limited to the eX- act method of constructionand apparatus shown so long as the characteristics are confined to theprinciples embodied herein. A full range of equivalents is anticipatedto the extent indicated in the following claims.

That I claim is:

1. In an amplifier circuit a plurality of vacuum tubes connected intandem, the inductances of the input circuits of said tubes beingconductively connected and inductively coupled and the inductances ofthe output circuits of said tubes being conductively connected andinductively coupled, and a single counter feed-back neutralizing circuitinductively coupled to both said input and output circuits.

2. In a radio frequency circuit, a plurality of tandem connected vacuumtube circuits each containing, inductance and capacity, a part of saidcapacity inherently arranged to produce a feed-back action, a singlecounter feed-back circuit having its total impedance equal to theaverage impedance of said circuits and its capacitance equal to the sumof the feed-back capacitances of said circuits.

3. In a radio frequency circuit, a plurality of tandem connected vacuumtube circuits each containing, inductance and capacity, a

ypart of saidcapacity inherenetly arranged to produce a feedback action,a single counter feedback circuit having its total impedance equal tothe average impedance of said circuits and its capacitance equal to thesum of the feed-back capacitances of said circuits, said capacitancebeing arranged in two parts.

4. In a radio frequency circuit, a plurality of'vacuum tube circuits ofsubstantially equal electrical constants arranged in tandem, saidcircuits having a plurality of paths through which feed-back currentsmay flow, and a single counter feed-back path coupled to each of saidcircuits and arranged to conduct an amount of counter-feedback energyequal to the amount of energy conducted over all of said feed backpaths.

5. In a radio frequency circuit, a plurality of vacuum tube cii'cuits ofsubstantially equal electrical constants arranged in tandem, saidcircuits having a plurality of paths through which feed-back currentsmay flow, and a single counter feed-back path coupled to each of saidcircuits and arranged to conduct an amount of counter-feedback energyequal to the amount of energy conducted over all of said feed-back pathsand a pair of condensers 'for insulating different portions of saidcircuits.

6. In a radio frequency circuit, a plurality of vacuum tube circuits ofsubstantially equal electrical constants arranged in tandem, saidcircuits having a plurality of paths through which feed-back currentsmay flow, a single counter-feedback circuit having a total of impedancesufficient to cause a flow of current at such a voltage and power factoras to cause sufficient energy to iiovv to counteract the energy appliedto the feed back circuit, said counter-feedback circuit being coupled toeach of said circuits.

7. In a push-pull amplifier circuit7 a pair of electron tubes havingtheir input circuits inductively connected and inductively coupled andhaving their output circuits conductively connected and inductivelycoupled, an independent inductance in each circuit and an independentcounter-feedback circuit di- Y into two symmetrical parts by means ofconi densers, one of said part-s being coupled to the inductance in saidinput circuit and the other of said parts being coupled to theinductance in said output circuit.

9. In a push-pullamplifier circuit, a pair of electron tubes havingtheir input circuits conductively connected and inductively coupled andhaving their output circuits con-v ductively connected and inductivelycoupled, an independent inductance in each circuit and an independentcounter-feedback circuit divided into vtwo symmetrical parts by means ofcondens-ers, the terminals of each being cont nected to the terminals ofcounter-feedback coupling coils, one of said coils being inductivelyrelated to the inductance in said input circuit and the other of saidcoils being inductively related to the inductance in said output`circuit.

10. In a push-pull amplifier circuit, a pair of electron tubes havingtheir input circuits conductively connected and inductively coufeo pledand having their output circuits conductively connected and inductivelycoupled, an independent counter-feed-back circuit comprising a pair ofcounter-feed-back coupling coils and a pair of condensers, said pair ofcondensers dividing said circuit into two symmetrical parts, theterminals of said condensers each being connected to the terminals ofsaid counter-feedback coupling coils, the relative proportions betweenthe inductance and capacity of said counter-feedback circuit being soadjusted as to counteract to substantially the same degree all feed-backcurrents within the amplifier circuits.

ll. In a push-pull amplifier circuit, a pair of electron ytubes havingtheir input circuits conductively connected and inductively coupled andhaving their output circuits conductively connected and inductivelycoupled, an independent counter-feed-back circuit comprising a pair ofcounter-feed-back coupling coils and a pair of condensers, saidcondensers dividing said circuit into two symnietrical parts, theterminals of each of said condensers being connected to the terminals ofcounter-feedback coupling coils7 the relative proportions between theinductance and capacity of said counter-feedback circuit being soadjusted as to counteract to substantially the same degree all feed-backcurrents within the amplifier circuits and automatically compensatingfor variation in coupling" losses by increased energy conductivity.

12. In a radio frequency amplifying circuit, a pair of electron tubeseach having grid, plate and cathode elements, circuits interconnectingsaid elements, impedances, resist! ances and Vcapacities disposed insaid circuits, means for neutralizing the undesirable feedback currentsin said circuits consisting of an independent counter-feed-back circuitcoupled to the electron tube circuits wherein saidcircuits have thefollowing proportions:

where A is the impedance of the input cir "ilo cuit, B the impedance ofthe neutralizing coil coupled with the input circuit, b the impedance ofthe neutralizing coil coupled with the output circuit, C the inherentcapacity of the feed-back circuit and o the capacity of thecounter-feedback circuit.

13. The method of preventing the output circuits of a push-pullamplifying stage from a'ecting the input circuit of said stage, whichcomprises balancing the retroactive eifects in one tube by impressing onits grid a voltage opposite in phase to its alternating plate voltage,and balancing the retroactive effects in the other tube by transferringa voltage from its plate to a point in its grid circuit of oppositealternating voltage to its grid, through a single coupling common to thetwo tubes.

lll. A circuit comprising two space discharge devices connected inbalanced or pushpull relation, each having a cathode, an anode and agrid or impedance control element, and a single, common connectionbetween the anode and grid circuits of said devices for neutralizing thegrid-plate capacities of both of said devices.

15. A circuit for capacity-neutralizing a push-pull vacuum tube stagecomprising a capacity connected between the anode of one tube and thegrid of the opposite tube and of such magnitude as to neutralizesubstantially the retroactive effects in both tubes.

16. A circuit for capacity-neutralizing a push-pull vacuum tube stagecomprising a connection from the anode of one tube to the grid of theopposite tube, said connection including capacity of a value tosubstantially prevent retroactive effects in both tubes.

17. In an amplifier comprising an even number of space discharge devicesconnected in balanced or push-pull relation symmetrically about theircommon cathode connection, a capacity path joining points in the outputand input circuits on respectively opposite sides ofthe common filamentconnection, said path serving in common to neutralize eectivelyinter-electrode capacity effects in both halves of the circuit.

In testimony whereof I aiiix my signature.

EDWARD W. FEARING.

